This invention relates generally to machine control. In particular, this invention relates to control of linear motion of a single rigid machine member driven by two independent servo mechanisms.
Machines incorporating gantry structures have been traditionally used for milling and drilling large components with extreme precision. In recent years gantry structures have been adopted for use in program controlled robots and manipulators for material handling between multiple machines and for large workpiece processing. Propulsion of a gantry is effected at supporting ends and to achieve the desired linear motion of the gantry relative to the machine frame without skew of the ends, it is necessary to drive the gantry ends at the same velocity. A degree of velocity matching may be achieved mechanically using a single driving motor and a drive shaft connecting the propulsion elements at the gantry ends. Such mechanical approaches are, nevertheless, subject to skewing caused by twisting and bending of the drive shaft. It is preferable to provide separate driving motors and transmissions for each gantry end. Servo mechanism control of the driving motors assures velocity matching within the tolerances of load matching at the motors and the servo mechanism control components. The linear motion of the gantry is controlled by a single motion axis command which is applied simultaneously to both servo mechanisms. This type of control shall be referred to herein as split axis control. Split axis control is susceptible to skewing between the drives attributable to differences in load or unmatched variations in servo mechanism components. Heretofore, such skewing has been accommodated by monitoring servo mechanism feedback signals and reducing velocity or stopping motion when the skew becomes excessive.